The present disclosure relates to a grinding machine device with a grinding spindle unit, which has a motor-driven drive shaft mounted therein and a grinding wheel fastened to one end thereof, and with a pivotable mounting of the grinding spindle unit on a receiving part of the grinding machine, wherein, due to the pivotable mounting, different inclinations of the drive shaft with respect to a reference line are set.
The present disclosure also relates to a method for pivoting a grinding spindle unit, which is connected via at least one pivot axis to a receiving part of a grinding machine and has a motor-driven drive shaft and also a grinding wheel fastened to one end thereof, whereby, as the grinding spindle unit is pivoted, different inclinations of the drive shaft with respect to a reference line are set.
According to the prior art, the pivotable mounting of grinding spindle units of this type located on the grinding machine consists in the fact that the grinding spindle unit is pivoted together with the receiving part as a whole, that is to say with its entire housing and the respective drive motor, in a motor-driven manner about a pivot axis with respect to the rest of the grinding machine. Here, the pivot axis is formed as a conventional pivot axis within the context of mechanical engineering. In other words, the pivot axis can be formed structurally specifically in the form of a bearing shaft or can be defined geometrically exactly by the center axis of mounts in which the entire grinding spindle unit is mounted. By pivoting the grinding spindle unit, the grinding wheel thereof can be leaned against the workpiece at different angles, and different grinding wheels can also be used if a plurality of grinding spindles are attached to a pivotable receiving part of the grinding machine.
With the known design of round/unround universal grinding machines, the pivot axis of the pivotable grinding spindle units is often referred to by practitioners as the B-axis. The B-axis is preferably directed perpendicular to the drive shaft of the grinding spindle unit and is also arranged perpendicular to the plane given by the possible linear axes of displacement of the grinding wheel and/or workpiece, wherein these axes of displacement are referred to in practice as the X-axis and Z-axis. The aforementioned plane most commonly runs horizontally, such that the B-axis is directed vertically. According to the prior art, a motor actuator is normally used to pivot the grinding spindle unit, the control of said motor actuator being incorporated into the control and regulation device of the grinding machine as a whole. An example of the prior art described here is presented in DE 102 35 808 A1.
With these known devices for pivoting grinding spindle units, large pivots paths can be provided and numerous grinding tasks can be performed with a high level of accuracy. For example, the inclination of the grinding wheel can be changed constantly during running operation by means of the CNC control, wherein a movement in one or more linear movement directions is also performed simultaneously. Complex curved and/or inclined contours can thus be produced with a high level of accuracy and high surface quality. However, there are borderline cases in which the grinding machines with the grinding spindle units pivotable in the conventional manner no longer provide satisfactory results.
A borderline case of this type is the grinding of bearings that are located on shafts and that are to have a “contour” that deviates from the cylinder shape. This contour may consist of a slightly outwardly curved spherical contour, which is referred to by practitioners as a “ballus”. When mounting crankshafts for example, the deviation from the cylinder shape outwardly is in a range from 0 to 5 μm. With other shafts, such as camshafts, conical bearings or cams, that is to say the contour of a cone, may also be requested, as well as the contour of a double cone with a maximum in the axial center. Contours of this type can be produced economically, particularly with inclined grinding wheels.
Often, a cylinder correction has to be made on the workpieces to be ground because clamping errors are present. This problem occurs particularly in the case of large crankshafts, which are relatively soft structures and with which, in spite of all precautions taken after the clamping the shaft for grinding, not all main bearings run exactly in line with the relevant longitudinal axis of the crankshaft. Errors of this type have to be counteracted during the grinding process by a deliberately controlled correction deviation from the normal position. In these and similar applications, it is necessary for the longitudinal and rotation axis of the grinding wheel to be pivoted with a high level of accuracy by a very small angle with respect to the reference line. Here, the reference line is normally the longitudinal and rotation axis of the grinding wheel when this runs exactly parallel to the center line of a rotating workpiece to be ground.
Another case in which an inclination of the grinding wheel by a small, yet precisely set angle with respect to the workpiece is necessary concerns the external cylindrical grinding of rotationally symmetrical workpieces, wherein a cylindrically trued grinding wheel is guided with a small clearance angle against the workpiece surface to be machined and the grinding wheel bearing against the workpiece at the end face contacts the finished ground workpiece surface only over points, see DE 34 35 313 C2. This grinding method known under the trade name “Quickpoint” enables short grinding times in conjunction with low heat development and high rotational speed of the workpiece. A reliable adjustment of the clearance angle even in a small pivoting range may be advantageous if different grinding tasks are to be managed or an existing grinding machine is not to work continuously by this method.
In all applications mentioned here, the pivotable mountings of the grinding spindles, that is to say the conventional grinding spindle units adjustable in a motor-driven manner, are at their limits. The reason for this is that these grinding spindle units are relatively bulky due to the necessary grinding accuracy and also due to the necessary pivoting device with bearings and drives. The movement of these large masses again requires large drives, such that the inertia on the whole reduces the displacement speed and the adjustment accuracy. The normal bearing of the grinding spindle units, which is sufficient for the large pivot paths when pivoting about the B-axis, is no longer sufficient for the fine adjustment in the above-mentioned cases. A highly accurate B-axis that must run without play and without friction is required. An approach for improvement could consist in forming the pivotable mounting of the grinding spindle units hydrostatically about the B-axis. This solution would be very costly however and could lead to grinding machines that are complicated to operate.
WO 2008/075020 A1 contains a proposal of adjusting a narrow grinding wheel of circular peripheral contour during operation such that its peripheral surface provided with the grinding coating is placed at different angles against the workpiece to be ground, although the position of the rotation and drive axis of the grinding wheel remains unchanged. For this purpose, the region close to the center of the narrow disk-shaped grinding-wheel central part of the grinding wheel is fixed between two securing flanges. Of these, the securing flange located on one side of the grinding wheel central part has a greater diameter than the securing flange located on the other side of the grinding wheel central part. The distribution of the rotating masses is therefore asymmetrical. When this grinding wheel is rotated, it deforms reproducibly with increasing rotational speed from a planar circular plate to the shape of a dish or a flat bowl, wherein the securing flange of greater diameter is located inwardly on the base of the bowl. Due to this deformation, the circular peripheral contour of the grinding wheel adopts an inclination; the size of the angle measured in an axial plane compared to the starting position with unmoved grinding wheel is dependent on the selected rotational speed.
The displaceable grinding wheel according to WO 2008/075020 A1 makes it possible, merely by changing the rotational speed, to grind outwardly curved bearing seats using a concavely contoured grinding coating, the axial extension of said bearing seats being broader than that of the grinding coating, without having to incline the drive shaft of the grinding wheel. Similarly, conically shaped cams of alternating direction of inclination can also be ground on a camshaft using a grinding coating that has a rectangular profile.
A disadvantage of the proposal according to WO 2008/075020 A1 is that the link between the inclination of the grinding wheel peripheral surface and the rotational speed is dependent on numerous parameters, such that an individual characteristic curve has to be created for each grinding wheel. In addition, the geometry of the grinding point influences the rotational speed actually to be set in grinding operation, wherein an amended rotational speed changes the preselected inclination unintentionally. With unequal loads, rotational speed fluctuations are also unavoidable, which may likewise influence the grinding result unfavorably. A further disadvantage may be that the rotational speed optimal for the shape-changing effect of the grinding wheels is often different from a rotational speed necessary for an optimum grinding result. In order to reconcile these two parameters at least approximately, specific purposeful changes would have to be made to the grinding wheel body, whereby a greater number of grinding wheel types is ultimately necessary.